Considerable attention in recent years has been directed at the problem of the formation and emission into the atmosphere of highly toxic chlorinated organics by municipal and industrial waste incinerator plants. One of the major impediments to increased construction of such plants for the incineration of municipal and industrial waste has been the concern over the formation of such toxic chlorinated organics, namely polychlorinated dibenzodioxin (dioxins) and polychlorinated dibenzofuran (furans). It is desirable to prevent the formation of toxic chlorinated organics, which will 1) eliminate toxic air emissions, 2) eliminate disposal of toxic ashes, and 3) minimize the need for wet or dry scrubbers, which are presently used to remove acid gases and chlorinated organics from flue gas emissions.
Control of combustion factors such as temperature, overfire/underfire air ratio, carbon monoxide (CO), oxygen, and waste feed moisture, all of which have been shown to be related to the formation of dioxins and furans, can minimize the formation of these chlorinated organics. However, complete or effective elimination by this method appears impossible due to the complexities involved in monitoring and regulating the combustion of municipal and industrial waste. While it is generally agreed that control of combustion can minimize the emission of large amounts of toxic organics, complete or adequate control and destruction of dioxins and furans in waste incinerators cannot be ensured by these means alone. Indeed, there appears to be some dispute regarding the effectiveness of combustion controls, such as high furnace temperature, on the destruction of organics and the reduction of dioxins and furans.
Other methods of controlling toxic organics include sorting and removing chlorine-containing wastes, such as polyvinyl chloride (PVC) plastics, prior to waste combustion. Since chlorine-containing plastics account for only about 50% of the chlorine content of municipal solid waste (MSW), even complete removal and separate disposal (such as by burying in a landfill) of chlorine-containing materials will not sufficiently eliminate the problem. Indeed, this method appears to have only an erratic correlation with the reduction of toxic organics.
Spray drying by flue gas injection of a sorbent slurry is another method of toxic organics reduction. Toxic organics and their precursors tend to condense and become absorbed on the dried sorbent particles. The sorbent and flue gas particles are then removed by a baghouse or an electrostatic precipitator, reducing the toxic organic gas emissions. However, the collected particles will contain organic (e.g. dioxin/furan) levels that may be of considerable concern to subsequent solid waste disposal. Further, the procedure may promote the formation of dioxin/furan. As the sorbent/ash particles are trapped on the surfaces of the particulate collection device, their residence time in the system increases. This increases the amount of time that the condensed dioxin/furan precursors remain at temperatures which may be optimal for such formation.
Other methods have been suggested to prevent or minimize the formation of toxic emissions, as contrasted with emission removal after formation. These include, for example, U.S. Pat. No. 4,681,045--Dvirka et al. directed at injecting sodium carbonate (Na.sub.2 CO.sub.3) or sodium bicarbonate (NaHCO.sub.3) into the furnace to react with chlorides to form sodium chloride (NaCl) for reducing acid gas emissions such as hydrogen chloride (HCl) and sulfur dioxide (SO.sub.2): thereby preventing downstream formation of dioxin/furan. However, this patent teaches against the use of calcium compounds such as calcium oxide (CaO) for such acid gas removal purposes. CaO is suggested in U.S. Pat. No. 2,800,172 for addition to petroleum fuel oil being burned to limit slag formation. However, these patents neither teach the process claimed herein nor achieve the desired objective of the present invention.